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  <title>Document</title>
  <style>
    *{
      padding:0;
      margin:0;
    }
    /* 父子盒子嵌套外盒子折叠
    解决：BFC   第四个渲染规则    BFC的区域 会计算子盒到父盒子margin-top的距离
    
    */
    .box{
      width:200px;
      height:200px;
      background-color: green;
      display:flow-root;
    }
    .son{
      width:100px;
      height:100px;
      background-color: red;
      margin-top:20px;
      
    }
  </style>
</head>
<body>
  <div class="box">
    <div class="son"></div>
  </div>
 
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